A Tainted Soul's Power
The weeks after the incident in court were blurred for Luna, she could remember pain and hate, the two things that she had felt for years. She only remembered one night where she actually felt content, she was preparing her little secret and enjoying some Earl Gray tea, it was delightful compared to her otherwise bland life.
Celestia still did not suspect a thing, she was far too engrossed with her royal duties to notice Luna's festering distaste for the citizens of Equestria.
It wasn't until those magical words were spoken by the creature in Luna's mind that anything changed drastically. We are ready. It is time to end Celestia's reign.
"Excellent. We will rise. Tell us, how do you intend to go about such a plan? Thou has promised us limitless power, yet, you have not told us how thy plan to do so. Perhaps... possession? We have no issue with such a plan, as long as we achieve a glorious victory." I suppose my plan is possession. You must truly desire victory, to be accepting of such an idea.
Luna's pure excitement for finally being able to act out against Celestia caused her magic to attack her body, it made her bleed once more. However, she did not care for her physical pain any longer, only her emotional pain. Her joyous trembling was interrupted by Celestia entering the room with a plate of tea and cookies. It was a split second before Celestia was holding Luna's face and bombarding her with questions.
"Was happened, dear Sister?!" Her thorough examination didn't show any impact damage, but something radiated from the blood that was strikingly familiar. Luna. "Luna, did thou do this to thyself? I can feel thy magic." Luna hissed in a bit of pain and shock, before she pushed Celestia away with force. "Leave us, Sister. Thou was not supposed to see this. We will explain to thou later." Celestia felt a wave of energy that came from Luna's horn, it was a signal that she was being truthful, it was enough to convince her to begrudgingly leave.
Be careful next time. We almost got caught. She knows too much now. "Yes, I know. Let us continue as planned. Tonight, we must not give her the time to plan."
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Celestia paced around the throne room, she cancelled her evening appointments and her mind was racing with thoughts and questions. Her relief was short-lived when Luna finally walked into the throne room, her body shook with every step and her mane seemed dead at the side of her head, her nose and mouth were dripping with blood at a much more disturbing rate. However, something else about her appearance made Celestia freeze in fear, her horn, it was curved, just like Sombra's. "Luna, dear Sister. What has happened to you?" The elder asked, the younger only had a few words in reply, "Why do you care?"
Luna walked through the throne room, her body slowly became more stable and her mane began to flow once more. "We care not for your pity. You have done enough damage, Celestia." As Luna stood in front of the throne, her coat darkened and her horn glowed a bright blue. A black aura surrounded the moon princess and she laughed gleefully. "N-NO!" Celestia cried out in a wild panic, she attempted to move, but something rooted her in place, she looked down to see bright blue chains around her hooves.
"You are a fool, Celestia. You made Luna suffer, and now she is fighting back." The aura disappeared and something otherworldly was left in its wake. Her coat was as black as darkness itself, her mane flowed without pattern, her wings were bat-like, her eyes were like a dragon's, whatever she was, it was not Luna. "Oh, come now, Luna wanted this. I will not let her down." The creature whipped around and unleashed a beam of magic from her horn, the wall exploded from it. "The night will be eternal, the ponies forever free from your tyranny!" Her magic glowed bright and the moon rose at its master's command, it rose until it blocked the sun, creating an eclipse. "What are you doing?! Lower the moon, it is your duty!"
"Is it, Celestia? I am not your little Luna. I am something far superior, a creature made from her pain and anger and sadness. I am Nightmare Moon."
Celestia found herself trembling in terror. "Aww, is little Tia afraid of us?" Nightmare chirped out and stepped towards the alabaster alicorn. "Luna, please-" Nightmare stomped loudly, cutting of Celestia. "Silence! I am not Luna, you bitch. I am something far worse, I am your worst fears and nightmares, Luna's suffering, I am devil incarnate."
With a jolt of movement, Celestia overcame her chains and opened her wings. She had to challenge the dark mare. "I do not want to fight you, but I will if I must. Do not make me do this, Nightmare." The alicorn in question rolled her eyes and fired a beam at Celestia. "Oh, enough with your high moral ground speeches! You are not the hero, you are the villain. You took everything from Luna and I am here to take it back." Celestia dodged the beam of energy with a split second to spare, she fired off her own before taking off, attempting to flee the battle long enough to plan an actual strategy. Nightmare easily snaked by the attack and was already on her tail, only getting closer.
It took intense focus to both remain ahead of Nightmare and dodge her attacks, too much, Celestia couldn't think, couldn't plan.
Suddenly, there was a sharp pain in her side, the smell of burning flesh for a moment before she was going down. Celestia attempted to open her wings, but it only caused a wave of agony and nausea. She hit the ground. Everything went dark for a minute before she opened her eyes again, she saw Nightmare laughing in the sky. Celestia stood slowly, her side burned but she endured it, her mind cleared and she knew what she had to do. Her magic lit up.
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Nightmare felt pride, she had done it, she had gotten revenge for Luna, she knocked that incompetent whore out of the sky. Her happiness was interrupted by a bellowing voice that practically shook the earth below her.
"NIGHTMARE!"
"Damn it. Why can't she just fucking die already?!" Nightmare huffed angrily and whipped around to see a bright glow emanating from the hole in the roof of the castle. She has the Elements. We must be prepared, do you have the spell ready? Luna asked within her mind, Nightmare laughed. "Yes, I do. We'll stick to the plan, she has not a single idea that we are ready for her old trick."
Celestia flew up out of the hole with the Elements of Harmony around her, they were charging up, but so was Nightmare's horn. "I am sorry, Luna, but she has left me with no choice."
The elder alicorn released a sob as she fired. However, Nightmare fired as well, the pitch black beam went straight through the Elements' energy. It hit the Element of Loyalty. The crystal darkened and the energy dissipated instantly. Celestia's terror was evident as the pure black magic linked with the Element of Honestly and Laughter. "You are a true idiot, Celestia. We knew of your pitiful plan. How dare you attempt to turn our Elements against us."
"N-no... This is i-impossible." Celestia stuttered out, her mane drooped and her pink-tinted body paled completely, making her look like a ghost. "Your reign ends here. It is time for you to get a taste of your own damn medicine, you bitch." The other three Elements obeyed Nightmare, she drew them over and they charged up again, but they were different, they radiated a bright blue and deep black energy. "DIE!"
The energy blasted and Celestia felt a tear slide down her cheek. It was over quickly. The energy dragged the alicorn to the deepest pits of Tartarus.
Nightmare grinned and landed on top of a hill.
She felt lonely. Do you want to separate? We can do that. Nightmare nodded and a piece of her mind went silent, Luna stepped out of her body. "Ah, the fresh night air, is it not a comfort in times like these? When you feel so lonely and it seems nothing will make you feel any better?" Nightmare sighed and nodded once more, her dragon-like eyes seemed to brim with tears, her body shook, Luna quickly embraced her. "What is wrong?!" Luna cried out in worry. "I am not sure, I just feel as though I have not done enough for you. As if I have failed you." Nightmare's voice was no longer strong and confident, it sounded weak and terrified. "You have done more than enough for me, I can never thank you enough, Nightmare. Shed your tears, let it all out. You worked hard and deserve rest."
So, Nightmare cried for what felt like hours, though only minutes, before her heaving breaths evened out and her tears stopped flowing. "Thank you, Luna. I suppose it is time for us to rest. Tomorrow, we must continue our preparations for our rule."
They fell asleep on the hill, bathed in the silvery moonlight.